The setting for the Sanctus XVII dates back to the eleventh century, according to the Gregorian Missal. It is in mode V, the medieval Lydian mode, with its dominant on DO and its final on FA (see page 176 in The Parish Book of Chant) and its scale can be played on: FGABCDE [NOTE: I use medieval/modern names for the modes rather than the classical greek names..see wikipedia for more information].